1. Technical Field of the Invention
This invention relates to mobile telecommunications systems, and more particularly, to a method and system which allows support of local control functionality in a mobile telecommunications system independent of mobile station roaming status.
2. History of the Prior Art
In a cellular telecommunications system the user of a mobile station communicates with the system through a radio interface while moving about the geographic coverage area of the system. The radio interface between the mobile station and system is implemented by providing base stations dispersed throughout the coverage area of the system, each capable of radio communications with the mobile stations operating within the system.
Existing cellular telecommunications systems operate according to various air interface standards which assure the compatibility of equipment designed to operate in a particular system. Examples of these standards include the EIA/TIA-553 Mobile Station-Land Station Compatibility Specification (AMPS), the IS-54-B Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard, and the EIA/TIA IS-136 Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard. Each of these standards provides specific details of the processes that take place in the mobile stations and base stations of a system in all modes of operation, including during its idle state, during rescanning of control channels, during registration, and during connection to voice or traffic channels. Each standard contains a very comprehensive level of detail regarding the operation of mobile stations and base stations. This comprehensive level of detail ensures that end-user equipment built according to a particular standard will be able to operate properly with any cellular system equipment also complying with the same standard.
The major cellular telecommunication systems standards are commonly established through consensus among the responsible government agencies and various members of the cellular industry. Cellular system vendors, cellular mobile station manufacturers and cellular network operators all have input into establishing the rules for interoperability of cellular systems and end-user equipment. The development of cellular standards is generally a slow process. The final standard will generally be of high quality, but will nevertheless be a compromise between the various solutions preferred by the different members of the cellular industry.
In a cellular system, each separate geographic area or cell of the system is illuminated by the radio transmissions from the base station which covers that cell and which is in turn coupled to a mobile switching center ("MSC") . The MSC controls the radio linkages between many different base stations and their associated cells. Each cellular system is made up of one or more MSCs and its associated base stations. A mobile station is typically assigned one system as its home system. When a mobile station travels from the geographic area covered by its home MSC, or the group of MSCs forming its home system, it is said to be "roaming".
A cellular phone system tracks the location of a roaming mobile station through the process of registration. In registration, a mobile station transmits a registration request message on the control channel of the base station nearest to its location. The base station will transmit a registration confirmation message back to the mobile station on the control channel if the registration request is accepted. This confirmation message confirms that the system in which the receiving base station is located has registered the mobile station in the system location area which that base station serves. A cellular system can be made up of one or more system location areas, and a system location area can be served by one or more base stations. The registration process can be either periodic or location-based.
Periodic registration occurs independently without mobile station user action, and is done periodically at predefined time intervals. The system periodically transmits certain parameters to mobile stations on the control channel of the base stations serving the cells in which the mobile stations happen to be located. The mobile stations then transmit registration request messages to the system, as they move about the system, at time periods calculated according to these parameters. A registration access message is received by the system at the base station serving the cell in which a particular mobile station is located at the time of transmission of the registration access. Upon receipt of the registration access message, the system registers that particular mobile station in the system location area containing the cell of the base station which received the registration access. After the mobile station has been registered the base station then transmits a registration confirmation message back to the mobile station.
Location-based registration occurs as a result of a mobile unit moving from one system location area to another or from one system to another. A base station will periodically transmit system location identifying data in an Overhead Message Train (OMT) on its control channel. The location identifying data includes a system identification value (SID, in EIA/TIA-553, IS-54-B or IS-136) which identifies the system in which the base station is located. In IS-54-B and IS-136 the location identifying data also includes a location area identification (LOCAID) which may identify a particular subdivision within a system. A mobile station periodically scans these base station control channels as it moves throughout the system or systems and receives the location identifying data for the system and/or system location area in which it is located. The mobile station compares the latest received location identifying data with data in its memory identifying the last system and/or system location area in which it received a registration confirmation message. If the two sets of identifying data match, the mobile is located in the system and/or system location area in which it is registered. If the mobile station has moved, and, the sets of data don't match, the mobile will transmit a registration access message which is received at the base station serving a cell contained in the system and/or system location area in which it is now located. The system will then register the mobile station in this new system and/or system location area and send a registration confirmation back to the mobile station. If a system is subdivided into system location areas, the new system location area may be in the same system in which the mobile was previously registered or in a different system.
Existing standards commonly allow for customization of the air interface to provide special local services within a system. Customization of the air interface may be used to provide additional services that are not implemented into a standard due to the lengthy standardization process, or that have been chosen not to be implemented to all, or that may be implemented according to parameters that are not the preferred choice of some cellular equipment manufacturers or operators. The standards define certain message codes that identify messages as local control messages. The content and use of these messages are not defined by the standards and are left to the discretion of mobile station and system equipment manufacturers. The local control feature of a system can be used to implement a mode of operation not defined in the system standard. Special services can be provided in this way.
The EIA/TIA-553, IS-54-B, and IS-136 standards all set some restrictions on the use of the local control services. Local control messages can be interpreted and acted upon by a mobile station only when the local control option is enabled by software in the mobile station. The provision of a software means to turn on or off the local control mode in the mobile station is mandated by each standard. In addition, the nature of the roaming feature provided by cellular systems inherently restricts the use of local control.
For local control purposes, the standards require a mobile station to compare the group identification of the SID received on the OMT of the control channel with a group identification of a Home SID permanently stored in the mobile, each time the mobile station registers in a new system. The group identification is comprised of a specified number of the most significant bits of the SID. Only if the bits of the Home SID that comprise the group identification match the group identification in the received and stored SID, will the local control function be enabled and local control messages be acted upon by the mobile. For this reason a mobile station must operate in a system location area within a system with a group identification the same as the group identification of its home system, or home area, to benefit from the use of local control. When a mobile station is not operating in a system or area which is part of the group identification, all benefits of local control are lost.
SID values for cellular systems have generally been set without regard of the possibility of using the group identification for local control purposes. In many cases, two cellular systems owned by the same cellular operator and providing service to two closely proximated cities will not have SID values with the same group identification. A mobile subscriber may roam between a mobile station's Home System and a nearby system and not be able to benefit from common local control features in the Home System and the nearby system, since roaming status is acquired within a short distance of the Home system or Home area.
Another problem with local control is that mobile stations are made by many different manufacturers. There may be no consistency as to which manufacturers build mobile stations that support local control functions that are implemented in a certain system.
In practice, given the multiplicity of cellular systems, the high mobility of many mobile stations which may roam between cities served by different systems, and the large number of mobile station manufacturers, the possibility of using local control is very restricted. Because of this, local control is rarely used.
It would be a distinct advantage then, to have a method and system which would allow a determination to be made as to whether roaming mobile stations can operate according to local control feature functions offered in a visited system. This would allow the mobile station user to take advantage of benefits offered by the local control features of the visited systems as the user moves from system to system.